Currently, photo-optical lithographic systems are widely employed in the fabrication of circuit pattern images on semiconductor substrates. Photo-optical lithographic technology, however, is approaching its ultimate limit, the point beyond which resolution cannot be improved due to diffraction effects. It has long been known, nevertheless, that considerable potential device speed and power enhancement is possible if higher resolution and improved processing techniques become available for production use.
Several new technologies are being developed in efforts to provide higher resolution lithography. Among these is electron lithography, in which the generation of a pattern is achieved by exposing a resist to energetic electrons, as opposed to the photons employed in photo-optical lithography.
Electron-beam lithography is the subject of extensive research efforts because of its unique combination of flexibility, resolution and alignment accuracy. An important element of this flexibility is the capability of fabricating devices without using physical masks, allowing design changes to be implemented and evaluated with minimum delay.
Among the first materials evaluated for use as electron resists were commercially available photo-optical resists. These materials offered the advantages of being readily available and the processing details for such materials were well understood. Such materials were less than satisfactory as electron resists, however, due to deficiencies such as poor electron sensitivity, low contrast, inadequate resolution and sensitivity to ambient light. For example, polymethylmethacrylate is a conventional photo-optical resist which is electron-sensitive, but its electron sensitivity is relatively low. Low sensitivity causes the required exposure time to dominate the throughput of electron lithography systems.
Because of the above, much research has been directed to finding new materials which would be suitable for use as electron-beam resists. Many such candidates have been reported in the technical and patent literature. U.S. Pat. Nos. 4,312,935; 4,312,936; and 4,338,392, for example, disclose a new proposed class of electron-beam resists based on conducting organic charge transfer salts, such as halogen salts of tetrathiafulvalene and their selenium analogs.
U.S. Pat. No. 3,936,429 discloses the preparation and use of reactive polymers formed by reacting homopolymers or copolymers of vinyl pyridine or substituted vinyl pyridine with certain unsaturated compounds to produce quaternary salt-type functional polymers. The resulting polymers are stated to undergo a reduction in solubility when exposed to light, electron-beams, and in general, electromagnetic waves, particle rays, thermal radiation, etc. Unfortunately, these polymers have been found to have poor photo-sensitivity in the deep U.V. range.
In order to fabricate complex integrated circuits at the lowest possible costs, it is desirable to combine photo-optical and electron-beam lithographic techniques in so-called hybrid lithography. This allows use of known photo-optical resists using established, inexpensive procedures for those steps in which the resolution of electron-beam lithography is not required. The process can be completed using electron-beam lithography to achieve submicron resolution where needed. However, in order to use hybrid lithography, resists are needed which have good sensitivity to electrons and light.